1. Field of the Invention
The present invention relates to a liquid crystal backlight device and a method for controlling the same and, more particularly, to a liquid crystal backlight device, which generates a modulated backlight activation signal to control the backlight illumination patterns, and makes use of stable illumination to improve the display quality.
2. Description of Related Art
A conventional liquid crystal display apparatus comprises a liquid crystal panel and a backlight module. The liquid crystal panel comprises a plurality of scan lines, a plurality of data lines, and a plurality of pixel elements. The backlight module is disposed behind the liquid crystal panel to illuminate the liquid crystal panel. The backlight module dominates the luminous quality of the liquid crystal panel. In the prior art, when scan signals are generated in turn on the scan lines, the data write cycle is not synchronous with the backlight illumination frequency. Because the response speed of liquid crystal is slower, a hold-type effect is generated to cause the hold type problem.
In order to solve the problem of flickering frame due to the asynchronous phenomenon between the backlight illumination frequency and the data write cycle, a liquid crystal apparatus having light quantity of the backlight in synchronism with writing signals has been proposed in U.S. Pat. No. 4,958,915, in which the backlight illumination frequency is adjusted to be in synchronism with writing signals. FIGS. 1A to 1C are clock diagrams showing the relationship between the backlight illumination frequency and the scan signal of a liquid crystal panel in the prior art. In FIG. 1A, “OFF” means the backlight is cut off or its brightness is decreased below a certain specified value, while “ON” means the brightness of the backlight is increased above a certain specified value by its drive circuit. FIG. 1B shows the control clock for driving the backlight module corresponding to the illumination brightness change of the backlight in FIG. 1A. FIG. 1C shows the periodic change of presence (Y) and absence (N) of the scan signal of the liquid crystal panel. As can be known from the figures, the backlight illumination frequency is controlled to correspond to the scan frequency of the liquid crystal panel in the prior art so as to accomplish a better display quality.
U.S. Pat. No. 6,693,619 disclosed a liquid crystal display apparatus comprising a liquid crystal module, a backlight module, and a control circuit for controlling backlight illumination. The control circuit controls the backlight illumination frequency to be in synchronism with the synchronization signal of the liquid crystal panel. When the liquid crystal module scans an image, the relevant backlight is cut off until the scanning is finished. The liquid crystal module can therefore successfully display the image, hence solving the hold-type effect of image display.
FIGS. 2A to 2D show frame images of the prior art. In FIG. 2A, the nth frame is displayed on a liquid crystal module 20. As shown in FIG. 2B, when the (n+1)th frame is to be displayed on the liquid crystal module 20, the upper half image is scanned, and the above control circuit for controlling backlight illumination turns off the backlight for illuminating the upper half panel while keeping the backlight for illuminating the lower half panel. Next, as shown in FIG. 2C, the lower half image is scanned, and the above control circuit for controlling backlight illumination turns off the backlight for illuminating the lower half panel while turning on the backlight for illuminating the upper half panel. The scan step of the image is thus finished. Afterward the whole image of the (n+1)th frame is displayed, as shown in FIG. 2D.
When there are a large quantity of dynamic frames (e.g., when watching a movie or TV program), the hold-type effect will occur in the liquid crystal display apparatus because of slower response of liquid crystal. Moreover, the backlight illumination frequency is not synchronous with the scan signal or the data write signal. In the above two disclosures, a control circuit is used to control the backlight illumination frequency to be in synchronism with the scan cycle or controlling the on/off state of the backlight light to solve the hold type problem. The present invention proposes a liquid crystal backlight device to solve the hold type problem due to the hold-type effect of liquid crystal occurred in the prior art.